The present invention generally relates to detecting devices for detecting abnormalities in tape travel, and more particularly to a device for positively detecting abnormalities such as stoppage of magnetic tape travel which is caused by mechanical and electrical abnormalities in a traveling path of a magnetic tape, abnormalities in a driving system for the magnetic tape, and the like within a magnetic recording and/or reproducing apparatus.
Generally, in a magnetic recording and/or reproducing apparatus which employs a magnetic tape, a mechanical abnormality may occur when the apparatus is in a tape travel mode in which the tape travels. Such a mechanical abnormality may occur when dew is formed on a member which makes contact with the tape and the tape sticks to this member, or when the tape runs off a normal tape path due to some cause and the tape is caught by a member such as a gear, for example. When such a mechanical abnormality occurs, the tape stops traveling. In addition, an electrical abnormality may also occur in the tape travel mode of the apparatus. Such an electrical abnormality may occur when a motor itself such as a capstan motor and a reel motor breaks down, or a motor driving circuit fails, for example. When such an electrical abnormality occurs, the tape travel also stops. It is necessary to detect the abnormality in the tape travel when the mechanical or electrical abnormality occurs, so as to change the mode of the recording and/or reproducing apparatus to a stop mode by a detection signal and prevent the magnetic tape, the motor, and the like from becoming damaged.
As a device for detecting such abnormality in the tape travel, there was a conventional detecting device comprising means for detecting rotations of a take-up reel and a supply reel, two counters, and three NAND circuits. In this conventional detecting device, rotation pulses having a repetition frequency which is in accordance with the rotational speed of the take-up reel and are detected by the means for detecting the rotation of the take-up reel, are supplied to a clock terminal of the first counter and a reset terminal of the second counter. Rotation pulses having a repetition frequency which is in accordance with the rotational speed of the supply reel and are detected by the means for detecting the rotation of the supply reel, are supplied to a clock terminal of the second counter and a reset terminal of the first counter. The first counter counts the take-up reel rotation pulses and is reset by the supply reel rotation pulse. The counted value in the first counter is supplied to the first NAND circuit. The second counter counts the supply reel rotation pulses, and is reset by the take-up reel rotation pulse. The counted value in the second counter is supplied to the second NAND circuit. Outputs of the first and second NAND circuits are supplied to the third NAND circuit, and an abnormality detection signal is produced from the third NAND circuit.
When the recording and/or reproducing apparatus is in the tape travel mode such as a recording mode, normal reproduction mode, slow-motion reproduction mode, and quick-motion reproduction mode and the magnetic tape is traveling in a normal manner, the take-up reel rotation pulse and the supply reel rotation pulse are supplied to the reset terminals of the first and second counters to reset the first and second counters. Thus, the counted values in the first and second counters are within a range of "0" and "14", and do not exceed fourteen. As a result, at least one bit among the four bits which represents the counted value constantly assumes low level, in both the first and second counters. Accordingly, when the magnetic tape is traveling in a normal manner, the output levels of the first and second NAND circuits are both high. Hence, the output level of the third NAND circuit which receives these high-level outputs of the first and second NAND circuits, is low. This low-level output of the third NAND circuit is obtained through an output terminal.
On the other hand, when a mechanical or electrical abnormality occurs and interferes with the tape travel, one of the take-up reel rotation pulse and the supply reel rotation pulse ceases. As a result, the first or the second counter assumes a counted value "15", and the output level of the first or second NAND circuit becomes low. Thus, the output level of the third NAND circuit becomes high, and this high-level output is produced through the output terminal as an abnormality detection signal which indicates that an abnormality was detected in the tape travel.
The conventional detecting device described heretofore detected the abnormality in the tape travel, according to the existence of the take-up reel rotation pulse and the supply reel rotation pulse. Thus, there was a disadvantage in that it was impossible to detect abnormality in the tape travel when the two reels are still rotating even after the capstan motor has stopped and the tape has stopped traveling. Further, a ratio N.sub.T /N.sub.S between the rotational speed N.sub.T of the take-up reel and the rotational speed N.sub.S of the supply reel undergoes a large change within a range of 0.3 and 27, for example, according to the diameters of rolls remaining on the reels. However, in order to detect the abnormality in the tape travel although the ratio N.sub.T /N.sub.S undergoes a change in such a wide range, it was necessary to reserve sufficient margin for the detecting value. Therefore, there was a disadvantage in that it took a long time to detect that an abnormality has occurred from a time when the abnormality had actually occurred.